The endothelial cell is topologically poised in the vasculature to sense and respond to a host of environmental signals, including reactive oxygen species (ROS). These chemically active molecules serve important roles in normal, homeostatic signaling; and their potential for oxidative injury is ameliorated by an elaborate system of antioxidant defenses. As the flux of ROS increases, the endothelial cell responds by enhancing protective mechanisms, leading to a state of compensated stress; when the flux of ROS increases further, these protective mechanisms are overwhelmed, leading to a state of uncompensated oxidant stress. In this program project application, five project leaders have come together to investigate mechanisms that underlie oxidant signaling and adaptation to oxidant stress in the endothelial cell in health and disease. Project 1 focuses on the mitochondrion as an important component of endothelial redox signaling; Project 2 addresses the role of glucose-6- phosphate dehydrogenase and its enzymatic product, NADPH, as key determinants of the thiol redox state in the endothelial cell; Project 3 examines the role of Foxo transcription factors in promoting resistance to oxidant stress in endothelial progenitor cells; Project 4 considers the effect of ROS dependent oxidative modifications of Ras on insulin signaling in the endothelial cell; and Project 5 tests the hypothesis that mitochondrial dysfunction and resulting oxidant stress contribute to endothelial dysfunction in human atherosclerosis. This conceptually cohesive program brings together five well established project leaders who have a long history of productive collaboration to focus on a key theme in endothelial cell biology. Using contemporary methods of cell and molecular biology, as well as genetic animal models and human studies, mechanisms will be dissected at the molecular and cellular levels and applied to mammalian systems. With the work proposed in this proposal, we hope to be able to identify novel approaches to understanding the biomolecular basis of oxidant signaling and the adaptive and maladaptive consequences of oxidant stress in endothelial (patho) biology and vascular disease.